Vacuum Gyro Systems
In the cockpit of an aircraft you will find two gyroscopic instruments that are driven by the engine mounted vacuum pump. Some aircraft use a pressure pump system and even older models (and some experimentals) use a venturi placed under or attached to one side of the fuselage as a vacuum source.
Aircraft with an EFIS system normally have a backup attitude indicator, either driven by air or electrical power.
The gyroscopic instruments are: attitude, turn and bank / turn coordinator and gyro compass. Some experimental VFR only aircraft will usually have the attitude indicator, but that really depends on the choice of the builder or owner of the aircraft.
The turn indicator is also used as backup and is therefore electrically driven and we will discuss that instrument too, but first we start with the power source of the attitude and gyro compass: the vacuum system.
The trend we see currently is to install EFIS glass cockpits in most aircraft. As a result, aircraft manufacturers now choose to install conventional gyro systems as a backup system in the cockpit, but these are mostly electrical driven types and a smaller version. Sometimes older airplanes are being converted to a hybrid form of cockpit where the mechanical gyro's are replaced when they fail.
Typical Vacuum Gyro System
A typical light aircraft gyro vacuum system consists of the following parts: in-cockpit air filter, suction gauge, attitude and directional gyro, pressure relief valve and the engine driven vacuum pump with air exhaust, as can be seen in the image to the right. Sometimes a warning light is installed and this illuminates when the suction drops below 4,5 inHg.
Air is drawn in by the vacuum pump through a fine air filter and it enters the instruments to drive the gyro rotor. The air is then directed on the rotor by a small nozzle and the rotational speeds are set around 20000 RPM. The air is then routed through hoses along a pressure relief valve and pump and is eventually vented overboard somewhere in the engine compartment. Make sure that air exit is not blocked.
This filter utilizes 0.3 μm (micron) filtration media providing superior filtration without undue air flow restriction, it has 3/8" installation tubes for routing hoses to the attitude and gyro compass. You may expect its life time to be around 500 hours, but there have been cases in which the filter lasted a lot longer. As long as the suction gauge shows a good indication, there should be no problem.
Pressure relief valve
Without this valve the suction would be too high and could damage the delicate nozzle and rotor system by letting the rotor spin at a RPM that is too high. Premature wear and instrument failures will then be the result. The suction should be kept within 4,5 inHg and 6,5 inHg.
As already mentioned above, the image here shows a 1" suction gauge which you commonly will find in light aircraft. It clearly shows the correct range for the suction to be in the green arc. This is so that vacuum gyro instruments may operate within their specifications for said reasons.
There are basically two possible sources for an aircraft to obtain vacuum power. They are as follows:
This is usually a dry vane carbon type air pump and it has a limited life span. You may expect that to be around 500 to 1000 hours and if they fail you will notice a slow drop in suction and gyros will slowly start to tumble in the instruments. This effect is especially noticable in the attitude indicator.
Mostly used on small VFR only aircraft as this tube supplies vacuum only when the aircraft already flying (sometimes propeller slipstream will have that effect too just by taxiing on the ground, but it wil be below the specified range). And as they are sitting in the airflow, they will produce drag (on the other hand a vacuum pump costs a tiny bit of engine power) and it can pick up ice disabling these instruments for IFR flight.
Electrically driven gyro's
These are more expensive, but the advantage is that the gyro can run at a higher RPMs and the instruments are completely dust sealed and will give a more stable indication and longer service life for the pilot.
There are a number of checks that must be done on a regular basis by the pilot: during runup and flight the vacuum pressure must be between 4,5 and 6,5 inHg (in the green). Some installations have a low vacuum warming light on the panel, this should be illuminated when the engine is not running or at a too low RPM for the relief valve to be able to regulate the suction properly.
When suction is above 6,5 inHg, the rotor RPMs will be too high and they may eventually suffer from bearing damage. You will see that the instruments will react very quickly, almost too lively to be normal. Suction values near the lower end (4,5 inHg) of the scale will result in lower gyro RPMs and possible tumbling and much slower response and lagging of the indicators.
When the vacuum pump begins to fail there will be a gradually drop in suction indication and RPM of the gyros. This may go unnoticed for while until the rotor RPM is too low and indication of either the attitude (will start tumbling) or gyro compass indication is erratically.
The vacuum system should be checked at least annually or every 100 hours for certified aircraft. It would be a wise decision to keep the same schedule with homebuilts too.